something odd with my Owon O-scope and hopefully someone can suggest a answer.

the scope is a PDS5022S. it has a self calibartion feature which i use from time-to-time. the scope doesnt read voltage correctly when i am reading the analog output of my circuit.

the circuit is a 555 that oscillates slowly around 1Hz 50%pwm. the output shown on schematic.

C1 is 22uF electrolytic
D1 is 1n4148
R1 << R2 << R3

so basically the voltage dividers give me the output voltages i want when 555 swings high and low.

weird thing is, my scope is reading the voltage oddly. when the 555 swings low the scope reads a voltage less than zero, and when it swings high it reads a voltage that is less than what my Fluke meter reads.

as example, if i set the output voltage (using Fluke) to +0.7v on the high side amd +0.1v on the low side, the scope reads -0.045v and +0.5v.

two things are odd. 1) the voltage swing isnt even correct on the scope. it should swing 0.6v, but the scope reads a delta of 0.545, so i thought maybe the calibration might be off, and 2) why the scope reads a less than zero voltage when 555 swings low.

so i re-ran the self calibration and then got a precision voltage reference IC which has several dc output voltages. weird, both my Fluke and scope read the exact same voltage on the IC. i also have on my desk a crude power supply, and both the scope and Fluke read the same voltages (dc and ac on the full wave rectifier).

...and yes, i did make sure the scope probe settings are all correct.

very weird. i am trying to figure out why the scope reads funny on this circuit. any suggestions? ...and yes, there are other parts connected to the 555, i didnt draw them for simplicity.

the circuit has 7805 for 5v regulation. power supply is a Mastech HY3005D-3 and feeds the 7805 with 14v dc.

everything is tied to a common gnd, which is the negative (not chassis) of the power supply.

i did some more testing. i was looking for common mode voltage on the supply gnd (the neg is not the same as chassis gnd). i was only able to see some common mode voltage while varying the output voltage, but it would die back to zero. most likely due to caps on the outputs.

then i started looking at the scope more closely. seems that the scope gets progressively less accurate with voltages under 5v, and changing the scale up (200mv/div is less accurate than the 100mv/div scale with voltages under 5v). the cursor lines on this scope seem to move real digital like (small steps with no in between, although i was able to place the cursor line in the middle of the trace line). down in the 0.1 - 1v range the scope reads off about 20-50mv. this explains the off reading on low voltage, but not the negative reading. interestingly enough, the scope did read fairly accurate probing the v reference from AD584 (maybe off 1-2mv on all voltages of 10, 7.5, 5, 2.5), while my Fluje read dead-on. it just seems odd.

i'm now wondering if the scope's self calibration is doing inaccurate calibration.

I like to check things with sine waves. The scope's calibration signal is virtually always a square wave, which is fine. But a sine wave can be measured with both the scope and an average responding meter, the amplitudes can be compared, and there's no worry about amplifier bandwidth.

Thus, I can set my function generator to output 0.5 Vrms and the scope and the DMM will both read it as 1 volt amplitude (the generator is calibrated for termination into 50 Ω). This lets me check the amplitude response of the scope and the DMM.

I'll use square waves when I'm using a wideband meter like the HP 3400.

I like to check things with sine waves. The scope's calibration signal is virtually always a square wave, which is fine. But a sine wave can be measured with both the scope and an average responding meter, the amplitudes can be compared, and there's no worry about amplifier bandwidth.

Thus, I can set my function generator to output 0.5 Vrms and the scope and the DMM will both read it as 1 volt amplitude (the generator is calibrated for termination into 50 Ω). This lets me check the amplitude response of the scope and the DMM.

I'll use square waves when I'm using a wideband meter like the HP 3400.

Click to expand...

hmmm, ok, i have afunction generator. i will meter and scope that too. but as i noted i did measure 60Hz coming off of a step-down transformer and both meter and scope read it the same.

but for my circuit, reading the voltage on such low square wave freq is almost the same as reading a battery. i can accept calibration for measurement being off, but why the negative reading?

I read your original description a bit more carefully and I see why it's confusing -- the data don't make sense. Your DC measurements indicate the scope's amplifier and the Fluke meter read the same voltages. Thus, you'd expect them separately to give the same measurements on the 555 circuit.

Something you might try: hook the Fluke meter and the scope to the 555's output at the same time. The effect of the loading of 1 to 10 MΩ impedances should be negligible. Then see if they read the same.

If they don't read the same, then the scope (or the Fluke) becomes more of a suspect. If they do read the same, then you know they're being consistent.

I assume you're using a scope probe. It might be worth removing that from the measurement and seeing if things change.

Are you getting the same behavior on both channels of the scope? If not, that might provide a clue.

I'd also ignore the scope's built-in measurements and calculate the measured values from the displayed trace and vertical amplifier gain.

I read your original description a bit more carefully and I see why it's confusing -- the data don't make sense. Your DC measurements indicate the scope's amplifier and the Fluke meter read the same voltages. Thus, you'd expect them separately to give the same measurements on the 555 circuit.

Something you might try: hook the Fluke meter and the scope to the 555's output at the same time. The effect of the loading of 1 to 10 MΩ impedances should be negligible. Then see if they read the same.

If they don't read the same, then the scope (or the Fluke) becomes more of a suspect. If they do read the same, then you know they're being consistent.

I assume you're using a scope probe. It might be worth removing that from the measurement and seeing if things change.

Are you getting the same behavior on both channels of the scope? If not, that might provide a clue.

I'd also ignore the scope's built-in measurements and calculate the measured values from the displayed trace and vertical amplifier gain.

Click to expand...

i have not tried channel 2 of the scope, will need to do that next. as for measurement w/ both scope and meter connected, yes, i had everything attached at the same time, i also took measurements separately, and all the tests w/ and w/o a 1meg ohm load on the output. i also tried taking measurements of supply across a 35ohm load. i varied the the voltage between ~0.5 all the way up to 14v and i only see discrepancy on the very low voltage.

i suspect calibration, or maybe the probe is to blame. i will use different probes and different channel to see if results change.

hmmm, definitely the scope. tested on channel 2 and it was fine. i looked over all the settings. at this point i was using battery power, but i re-ran the self calibration again, and like magic channel 1 seems to be fixed.

very very weird. i guess the black-hole stuff is what i get for investing in a Owon scope..... but to date it has served me well.